1. Field of the Invention
The present invention relates generally to diversity antenna placement, and in particular, to a method of determining an appropriate distance between two receiving antennas in a radio telecommunications base station (BS) to provide good diversity performance.
2. Description of the Related Art
A cellular radio telecommunications system divides the whole service area into a plurality of base stations cells, i.e., small service areas, and the base stations are centrally controlled through an exchange system. An antenna is implemented to transmit or receive radio signals throughout the service area in the radio telecommunications system. However, the radio frequency (RF) signal (voice or electrical wave) is susceptible to fading. Accordingly, the base stations employ more than one antenna to compensate for the fading over the communication channel via diversity. That is, the radio telecommunication system selects the optimal signal among the RF signals received through a plurality of receiving antennas in a complex propagation environment. Typically, a base station uses two receiving antennas spaced by a predetermined distance to achieve the space diversity. At least two antennas are separately placed at a prescribed distance to reduce the multipath fading of the received signals and to achieve a good space diversity performance.
FIG. 1 illustrates a schematic view of a conventional base station employing the diversity antennas. As shown in FIG. 1, the base station is comprised of a first antenna 110 for transmitting or receiving radio signals, a second antenna 120 exclusively used only for receiving the radio signals, and an indoor system 130 connecting the first antenna 110 to the second antenna 120. That is, the first antenna 110 serves the dual purpose of transmitting and receiving radio signals, and the second antenna 120 is exclusively used for the reception of radio signals. The distance between the receiving antennas is a very important factor in improving diversity performance in a radio telecommunications system. A diversity antenna placement method varies with the operation frequency band of the antenna, and the optimal antenna placement maximizes the diversity effect to overcome fading over the communication channel. In particular, the distance between the receiving antennas should be varied according to the operation frequency range of the antennas.
Most existing radio telecommunications systems use a frequency band of 900 MHz or 1.8 GHz. However, the antenna diversity technology of such system is not adaptable to a wireless local loop (WLL) system operating at 2.3 GHz band. As the RF signal experiences different phases of fading depending on the operation range of the frequency band, an optimal diversity antenna placement distance is necessary to bring good diversity performance at 900 MHz or 1.8 GHz band is not affective at 2.3 GHz band. Accordingly, the conventional diversity antenna placement method has its limitation in determining the optimum distance between the diversity antennas to achieve good diversity performance in a new radio environment that operates at a higher frequency band.